The Bayer-type alumina manufacturing process where the aluminum oxide content of bauxite is recovered with the aid of a digesting liquor containing sodium hydroxide.
A significant part of the production cost of alumina (about 15-25%) is due to the price of sodium hydroxide introduced into the Bayer-cycle in order to supplement the sodium hydroxide losses of the cycle. For this reason the reduction of the caustic soda losses and the highest and most efficient recovery of caustic soda from the sodium compounds formed during the process constitutes a constant aim of alumina factories.
The larger part of the sodium hydroxide loss is generally due to the so-called "bound" loss which depends on the amount of the sodium aluminum hydrosilicates formed from the reactive silicon oxide content of bauxite and on the amount of sodium titanates formed during digestion from the titanium oxide content of bauxite, respectively. The said compounds are carried to the dumping place together with the red mud.
Further sodium hydroxide losses are due to the so-called "dissolved" losses derived from the incomplete washing of red mud. These losses arise from the sodium hydroxide adherent to the red mud which leaves the cycle at the end of the red mud washing line together with the mud.
The active, so-called caustic sodium hydroxide content of the Bayer-cycle, which is effective from the point of view of the dissolving of aluminum oxide, is further decreased by the presence of carbonate type minerals in the bauxite; said minerals convert sodium hydroxide into sodium carbonate during the digestion of bauxite.
The carbonate content of bauxite is generally present in the form of calcite (CaCO.sub.3), dolomite [CaMg(CO.sub.3).sub.2 ] and siderite (FeCO.sub.3).
Under the digestion conditions used [210.degree.-250.degree. C., 140-250 g/l of Na.sub.2 O.sub.k --the formula Na.sub.2 O.sub.k designates the caustic sodium hydroxide content, expressed as Na.sub.2 O] the percentual dissolving of calcite amounts to 70-90% and that of dolomite to 80-100%.
Further sources of carbonate formation, while being of minor importance but not neglectable, are the organic substance content of bauxite, the carbon dioxide content of air, the flocculating agents, etc. From these substances sodium carbonate is formed in the digestion liquor.
In order to reduce the above sodium hydroxide losses several processes have been developed which have partly been used on an industrial scale, too. A common feature of these processes is that they are directed to the reduction of only one source of loss, i.e. either to the "bound" losses of the solid phase of red mud, or the "dissolved" loss or the "salt" loss (the amount of sodium hydroxide rmoved with the precipitated soda salt or vanadium salt).
The following methods are used for the reduction of the losses of sodium hydroxide bound in the solid phase of red mud:
Digestion by using an additive. According to this process additives or catalysts are added to the bauxite or the slurry to be digested. Burnt lime is the most well-known additive. According to a recent method iron-containing hydrogarnet is used as catalyst, which promotes the goethite-hematite conversion during digestion and posseses several advantages in the processing of bauxites containing goethite (increase of the alumina yield, technological advantages etc.). This process is disclosed in U.S. Pat. No. 4,091,071.
Causticization of red mud. According to this process lime milk is added to the washed red mud slurry, the mixture is heated for several hours, and the liquid phase is separated from the solid phase, generally by filtration. The diluted alkali solution thus obtained is introduced to one of the washing stages and the mud is led into a receiver.
According to Hungarian U.S. Pat. No. 149,730 red mud is treated with burnt lime and a mixture of sodium hydroxide and sodium carbonate under pressure at a temperature of 140.degree.-200.degree. C.
In order to reduce the "dissolved" losses on the red mud settling and washing lines in the underflower of the settling apparatuses the concentration of the solid substance is increased e.g. by adding synthetic sedimentating agents and the diluted sodium aluminate solution leaving the apparatus together with the red mud is recycled in a suitable manner after separation.
The following methods are used to decrease the amount of sodium carbonate being enriched in the cycle and to regenerate the sodium hydroxide from the sodium carbonate:
The soda salt separated from the recycled digestion liquor by filtration or centrifuging is treated with lime milk, whereafter the thus obtained alkali having a concentration of 40-100 g/l Na.sub.2 O is evaporated or introduced into the washing line.
According to a process in widespread use the alkali stream derived from one of the washing apparatuses is treated with calcium hydroxide. Thus, the dissolved sodium carbonate is converted into sodium hydroxide (U.S. Pat. No. 3,120,996).
The above procedures are more or less suitable for the reduction of the losses of "dissolved" and "bound" sodium hydroxide and for the regeneration of soda.
According to the additive-type digestion process used for the reduction of losses of "bound" sodium hydroxide only about 10-15% of the "bound" sodium hydroxide losses can be recovered because if a larger amount of calcium oxide is added during digestion, calcium aluminium hydrosilicates of unfavourable composition (low SiO.sub.2 -content) are formed which gives rise to secondary alumina oxide losses.
The causticization of red mud might enable the recovery of 70-80% of the "bound" Na.sub.2 O content; however 30-50% of the chemically recovered sodium hydroxide leaves the Bayer-cycle together with the filtered red mud. A further drawback of the causticization of red mud is that expensive filtration equipment and additional labor are required.
Causticization of red mud under pressure is used nowhere in practice because of the high energy demand and expensive apparatus required.
According to general practice the recovery of Na.sub.2 O bound in the red mud and that of inactive Na.sub.2 O enriched in the solution are carried out in separate operation steps.